1. Field of the Invention
The present invention relates to a wafer heating apparatus to be employed for heating mainly a wafer and a ceramic heater to be employed for the apparatus and for example to a wafer heating apparatus suitable for forming a semiconductor thin film on a semi conductor wafer, a liquid crystal substrate, a circuit board or the like and for forming a resist film by drying and baking a resist liquid applied to the water.
2. Prior Art
In semiconductor thin film formation treatment, etching treatment, resist film baking treatment and the like in semiconductor device fabrication process, a wafer heating apparatus is employed for heating a semiconductor wafer.
Conventionally, the wafer heating apparatus has been made to be a batch type apparatus for carrying out film formation treatment of a plurality of wafers. Along with the recent tendency to enlarge a wafer size from 8 to 12 inches, a sheet-fed manner for heating wafers one by one has been employed to increase the treatment precision.
In the sheet-fed manner, the number of wafers to be heated by one time treatment is lessened, so that wafer treatment time is required to be shortened. Accordingly, a wafer supporting member is required to be suitable for shortening the wafer heating duration and quickening the vacuum attachment, transportation, and vacuum detachment of wafers, and at the same time, the heating temperature precision of a heater is required to be improved.
As an example of such a wafer heating apparatus described above, Japanese Patent Publication No. 11-283729 discloses a wafer heating apparatus, as illustrated in FIG. 12, including main components of a support 31, a heat homogenizing plate 22, and a stainless steel plate 33 as a plate reflection body. The support 31 is a bottomed member made of aluminum and having an opening 34 with a circular cross-section shape in the upper side. In the center part of the support 31, three pin insertion holes 35 are formed to insert wafer supporting pins (not illustrated in the figure) into. By moving the wafer supporting pins inserted into the pin insertion holes 35 up and down, a wafer W can be transported to and from a transferring apparatus. Also, a conductor terminal 27 is soldered to the terminal part of a heating element (not illustrated in the figure) and the conductor terminal 27 is inserted in a hole 57 formed in the stainless plate 33. In the outer circumferential part of the bottom part 31a, some holes 36 for leading out lead wires are formed. Lead wires (illustrated in the figure) for supplying electric current to the heating element are inserted through the holes 36 and connected to the foregoing conductor terminal 27.
Nitride ceramics or carbide ceramics may be used for the ceramic material forming the heat homogenizing plate 22. The heating element is proposed to be resistor strips in a plurality of concentrically formed patterns, as illustrated in FIG. 13, to heat the heat homogenizing plate 22 by electricity application. A heating element 62 and electrodes 63 are formed in the heat homogenizing plate 22 and sensor installation holes 64 are also formed.
Heat Homosinizing
The heat homogenizing plate 22 of such a conventional wafer heating apparatus is required to precisely control the temperature distribution in the wafer W plane within a range of xc2x10.5xc2x0 C. Therefore, Japanese Patent Application Laid-Open Publication No. 8-70007 proposes heating treatment of a substrate, as illustrated in FIG. 14, to be carried out while keeping a wafer W parallel to a heat homogenizing plate 22 made of aluminum and equipped with a heating means or a cooling means at a constant distance from the upper face of the heat homogenizing plate 22 by supporting the wafer W with spherical supporting pins 59 set in recessed parts 58 formed in the heat homogenizing plate 22.
In such a manner, by holding the wafer W at a distance from the heat homogenizing plate 22, even if warping or the evenness of the wafer W relative to the heat homogenizing plate 22 differs, the wafer is prevented from contact with the heat homogenizing plate and accordingly the unevenness of the temperature distribution in the wafer surface is suppressed. Further, such a structure is applied to a conventional heat homogenizing plate 22, since the heat homogenizing plate 22 itself is thick, the temperature distribution caused in a heating element 25 can be moderated owing to the thickness of the heat homogenizing plate 22. Even heating is thus made possible. However, the heat homogenizing plate 22 made of aluminum has a problem that it takes a long time to carry out heating and cooling to a set temperature and also it takes long to response to the altered set temperature due to a high thermal capacity.
In relation to that, along with requirement of fineness of semiconductor wiring, especially a photosensitive resist film is required to be heated in a widely varying temperature range. In order to shorten the heating treatment for every wafer, the thermal capacity of the heat homogenizing plate has to be small and the temperature has to be altered fast. Further, sensitive temperature control is also required from the time of setting the wafer on the heat homogenizing plate to the time of completing the heating treatment. As a heating element formed on a thin ceramic heat homogenizing plate 22 having a high toughness and a high thermal conductivity is proposed to carry out heating.
However, in such a wafer heating apparatus described above, if the thickness of the heat homogenizing plate is made thin, the temperature distribution generated by the heating element is not sufficiently moderated and the temperature of the wafer W takes a long time to become even.
Japanese Patent Application Laid-Open No. 6852 (2001) discloses a method for solving the thickness unevenness caused in the heating pattern printing direction by controlling the sheet receptivity of a heat generating unit to be less than 50 mxcexa9/xe2x96xa1 and making the strands of the patterned heat generating unit have curved parts. It is disclosed that the optimum range of the thickness of the heat generating unit is 1 to 50 xcexcm and that of the width of the beat generating unit is 0.1 to 20 mm and as evaluation results by a thermoviewer, the temperature dispersion is reported to be improved to the degree of about 0.5xc2x0 C.
However, the thermoviewer has an temperature measurement error depending on the uneven hue of the surface of an object to be measured and the ambient environments and therefore is impossible to carry out measurement as precisely as required temperature precision. Today, the temperature precision is required to be that measured in the state a wafer is actually mounted. For that, a temperature-measuring wafer, which is a silicon wafer in which sensors such as thermocouples, temperature-measuring resistors and the like are buried tends to be employed as a means for carrying out the measurement. When the heater disclosed in the foregoing Japanese Patent Application Laid-Open No. 6852 (2001) is subjected to measurement while such a temperature-measuring wafer is mounted on the apparatus, it is found difficult to satisfy the temperature dispersion within 0.5xc2x0 C. Further, it is also found there exist points showing peculiar temperature values in the bent parts and gaps among patterns of the heating element formed for moderating the printing unevenness.
In the case of employing a conventional apparatus, even if heating is possible in a stable and saturated temperature state with even temperature distribution, when a wafer W cooled to a room temperature is mounted on a mounting face of the heating apparatus controlled to be at a prescribed temperature and heated, only the portions where the heat generation capacity is increased for even heating are sometimes heated quickly as compared with other portions to result in uneven temperature distribution during the transition time for heating. Further, only the portions where the heat generation capacity is increased cause resistance alteration quickly and cause a problem that the temperature evenness is deteriorated within a short time.
Moreover, it is possible to improve the temperature evenness along with increase of the heat capacity in a heat homogenizing plate by increasing the thickness of the heat homogenizing plate, however that results in decrease of the thermal response and insensitivity of the temperature detection of thermocouples and accordingly, leads to an adverse result in temperature dispersion during the transition time for heating.
Also, it is possible to assure the heat generation quantity corresponding to heat release by enlarging a heating element so as to set it closely to a support for supporting a heat homogenizing plate, however in such a case, the resistance of the heating element is changed quickly and the life of the heating element is shortened attributed to the mechanical load by contact with the support and effect of the thermal stress to the heat transfer to the support.
A structure in which a wafer is parted from a mounting face 53 of a heat homogenizing plate 52 by supporting pins may be employed, however in this case, the heat transmission from the heat homogenizing plate, a heat source, to the wafer is carried out by radiation heat from the entire body of the heat homogenizing plate and thermal conduction from the supporting pins in combination. If the radiation heat and the thermal conduction from the supporting pins are not in balanced, the wafer temperature sometimes becomes low in the parts of the supporting pins or contrary high. The heat transfer by radiation is affected by the radiation characteristics of the heat homogenizing plate 52 and the distance between the heat homogenizing plate and the wafer and the heat quantity by the thermal conduction alters depending on the thermal conductivity of the supporting pins. Such inconformity of the heat transmission manners results in occurrence of temperature difference-in-plane of the wafer and local change of the quality of a coating and uneven reaction of a resist film.
A object of the present invention is to provide a wafer heating apparatus capable of homogeneously heating the entire face of a wafer set on a mounting face at a prescribed temperature within a dispersion range of xc2x10.5xc2x0 C. by a heat homogenizing plate equipped with a heating element.
Another object of the present invention is to provide a heating apparatus provided with a transitional heating property capable of shortening the duration from the time of setting a wafer at an ordinary temperature on the heat homogenizing plate to the time the wafer reaches the aimed temperature or to the time the wafer is cooled to a prescribed temperature.
Another object of the present invention is to provide a wafer heating apparatus capable of increasing the temperature of the wafer surface as evenly as possible during the heating process of the wafer at an ordinary temperature mounted on the heat homogenizing plate.
Another object of the present invention is to provide a wafer heating apparatus capable of distributing temperatures on the wafer surface as evenly as possible by keeping the balance of the heat quantity between radiation heat from the heat homogenizing plate and thermal conduction from supporting pins.
The wafer heating apparatus of the present invention includes a heat homogenizing plate of a ceramic whose one main face is set to be a mounting face for a wafer, a heating element composed of a plurality of resister strips which are formed in the other main face or buried in the inside of the heat homogenizing plate, and electric supply portions to be electrically connected with the heating element in the other main face, wherein the heating element is capable of uniformly generating heat in entire surface of the heat homogenizing plate to produce even temperature distribution in the upper face of the heat homogenizing plate by defining the area ratio S of the heating element, the width P (mm) of the resister strips, and the gap G (mm) between the adjacent resister strips so as to satisfy specified relations.
In the present invention, the area ratio S is defined as S=S1/100 mm2: wherein S1 (mm) denotes a surface area of the resister strips in an optional portion of a 10 mm square in an effective heat generation area formed on the heat homogenizing plate. In the wafer heating apparatus of the present invention, arrangement of the resister strips in a heating element is so set as to satisfy the following relations:
0.15xe2x89xa6Sxe2x89xa60.85; 
0.3xe2x89xa6Pxe2x89xa66.71xc3x97S2+1.52; and 
0.3xe2x89xa6Gxe2x89xa66.71xc3x97(1xe2x88x92S)2+1.55. 
In the present invention, by adjusting the ratio of the outer diameter of the heating element to the outer diameter of the heat homogenizing plate within a prescribed range, and specifying the thickness of the heat homogenizing plate, the wafer heating apparatus is made capable of heating a wafer with even temperature distribution in the entire wafer surface during a temperature increase by holding the heat homogenizing plate on a casing.
In the case where Y is defined as the outer diameter B of the heating element divided by the outer diameter C of the heat homogenizing plate, and X is defined as the thickness in mm of the heat homogenizing plate, the thickness X (mm) may be controlled to be in a range: 2xe2x89xa6Xxe2x89xa68 and Y is within a range satisfying the following inequalities:
Yxe2x89xa70.02X+0.7; 
Yxe2x89xa7xe2x88x920.02X+0.9; 
Y less than xe2x88x920.02X+1.08; and 
Yxe2x89xa60.96 
In the case of satisfying these relations, the temperature distribution on the entire wafer can be suppressed to 10xc2x0 C. or lower during increasing the temperature of a wafer newly substituted on the heat homogenizing plate, and also the lifetimes of the heating element can be prolonged.
Further, the heat homogenizing plate of the present invention, which may be equipped with a plurality of supporting pins in the mounting face for supporting the wafer over, and apart from, the mounting surface, preferably may have 0.8 or higher of emissivity xcex5 to infrared rays of 8 xcexcm wavelength at 100xc2x0 C. or higher temperature.
The heat homogenizing plate may have the supporting pins of 0.05 to 0.5 mm in height projected above the mounting face. Thus, by optimizing the heat radiation from the heat homogenizing plate, the temperature distribution in the wafer can be made more uniform through heat conduction from the supporting pins and heat radiation from the mounting face in combination.